Recent years have seen a marked increase in interest in fuel cell for the generation of electric power. One area where interest is high is in the design of propulsion systems for vehicles. As is well known, a typical fuel cell combines hydrogen and oxygen to generate electricity which may then be used to power an electric motor which can be used to provide propulsion for a vehicle.
While such systems have held promise for many years, most have been out of reach from the practical standpoint in that they require the vehicle to carry hydrogen as the fuel. The provision of oxygen for the fuel cell reaction is not a problem in that it is readily available from ambient air. In any event, early proposals required that hydrogen be carried in liquid form or in gaseous form at extremely high pressures. In either case, the vessels for carrying the hydrogen were large, heavy and cumbersome in comparison to fuel tanks for vehicles powered by internal combustion engines.
Moreover, there was and is no infrastructure in place to provide for the fueling of vehicles with liquid hydrogen or hydrogen under high pressure to allow widespread use of fuel cells in vehicles. And if that were not enough, where liquid hydrogen is considered as the fuel, considerable expense in terms of equipment necessary to assure vaporization of liquid hydrogen so that it can be used by the fuel cell is a further drawback. Consequently, fuel cell systems to date have been non-competitive with conventional internal combustion engine propulsion systems.
More recently, in order to solve the above difficulties, there have been a variety of proposals of fuel cell systems employing a so-called reformer. Reformers are chemical processors which take an incoming stream of a hydrocarbon containing or hydrocarbon based material and react it with water to provide an effluent that is rich in hydrogen gas. This gas, after being further treated to rid it of fuel cell poisoning constituents, most notably carbon monoxide, is then provided to the anode side of a fuel cell. Ambient air is provided to the cathode side of the fuel cell. The oxygen in the air and the hydrogen in the anode gas are reacted to provide water and generate electricity that may be used to power a load such as an electric motor.
The reformer must receive the fuel and water in vapor form. Consequently, if the disadvantage of high pressure vessels associated with some pure hydrogen fuel cells is to be avoided, some means of carrying the fuel in a liquid form in a tank comparable to gasoline or diesel fuel tanks must be provided along with the means for vaporizing the water and the fuel prior to its admission to the reformer. While for many non-vehicular applications, the matter of vaporizing the water and the fuel may be handled relatively simply, the problem is much more difficult where the production of electricity by the fuel cell is expected to respond rapidly to a change in electrical load. In the vehicular context, this means that the fuel cell must respond rapidly to changes commanded by the driver of the vehicle through changes in the position of the fuel cell equivalent of a conventional gas pedal.
It has been determined that the rapidity of response of the fuel cell to a commanded change depends on the mass of water and fuel in the vaporizer that feeds vaporized water and fuel to the reformer. The greater the mass of fuel and water in the vaporizer, the longer the response time. Consequently, it has been determined that to be effective in fuel cell systems powering loads which require rapid response to a change in conditions that the mass of fuel and water in the vaporizer be held to an absolute minimum. To meet this requirement, it is highly desirable that the fuel and water side of the vaporizer have as small a volume as possible.
In vehicular applications, it is also highly desirable that the overall vaporizer be as small in size as possible in terms of volume and in weight. Bulk and weight are highly disadvantageous in that weight reduces the overall fuel efficiency of the vehicle and bulk reduces the load carrying capacity of the vehicle to the point that it is impractical to provide a vehicle that can compete with conventionally powered vehicles in use today. It is also desirable to achieve a very short system start-up time.
The present invention is directed to overcoming one or more of the above problems.